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The ACS Virgo Cluster Survey. VIII. The Nuclei of Early-Type GalaxiesThe ACS Virgo Cluster Survey is a Hubble Space Telescope program toobtain high-resolution imaging in widely separated bandpasses (F475W~gand F850LP~z) for 100 early-type members of the Virgo Cluster, spanninga range of ~460 in blue luminosity. We use this large, homogenous dataset to examine the innermost structure of these galaxies and tocharacterize the properties of their compact central nuclei. We presenta sharp upward revision in the frequency of nucleation in early-typegalaxies brighter than MB~-15 (66%=4.2 pc, with the sizes of individual nucleiranging from 62 pc down to <=2 pc (i.e., unresolved in our images) inabout a half-dozen cases. Excluding these unresolved objects, the nucleisizes are found to depend on nuclear luminosity according to therelation rh L0.50+/-0.03. Because the largemajority of nuclei are resolved, we can rule out low-level AGNs as anexplanation for the central luminosity excess in almost all cases. Onaverage, the nuclei are ~3.5 mag brighter than a typical globularcluster. Based on their broadband colors, the nuclei appear to have oldto intermediate age stellar populations. The colors of the nuclei ingalaxies fainter than MB~-17.6 are tightly correlated withtheir luminosities, and less so with the luminosities of their hostgalaxies, suggesting that their chemical enrichment histories weregoverned by local or internal factors. Comparing the nuclei to the``nuclear clusters'' found in late-type spiral galaxies reveals a closematch in terms of size, luminosity, and overall frequency. A formationmechanism that is rather insensitive to the detailed properties of thehost galaxy properties is required to explain this ubiquity andhomogeneity. The mean of the frequency function for thenucleus-to-galaxy luminosity ratio in our nucleated galaxies,=-2.49+/-0.09 dex (σ=0.59+/-0.10), isindistinguishable from that of the SBH-to-bulge mass ratio,=-2.61+/-0.07dex (σ=0.45+/-0.09), calculated in 23 early-type galaxies withdetected supermassive black holes (SBHs). We argue that the compactstellar nuclei found in many of our program galaxies are the low-masscounterparts of the SBHs detected in the bright galaxies. If thisinterpretation is correct, then one should think in terms of ``centralmassive objects''-either SBHs or compact stellar nuclei-that accompanythe formation of almost all early-type galaxies and contain a meanfraction ~0.3% of the total bulge mass. In this view, SBHs would be thedominant formation mode above MB~-20.5.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.

Virgo Cluster Early-Type Dwarf Galaxies with the Sloan Digital Sky Survey. I. On the Possible Disk Nature of Bright Early-Type DwarfsWe present a systematic search for disk features in 476 Virgo Clusterearly-type dwarf (dE) galaxies. This is the first such study of analmost-complete, statistically significant dE sample, which includes allcertain or possible cluster members with mB<=18 that arecovered by the optical imaging data of the Sloan Digital Sky Survey DataRelease 4. Disk features (spiral arms, edge-on disks, or bars) wereidentified by applying unsharp masks to a combined image from threebands (g, r, and i), as well as by subtracting the axisymmetric lightdistribution of each galaxy from that image. Fourteen objects areunambiguous identifications of disks, 10 objects show ``probable disk''features, and 17 objects show ``possible disk'' features. The numberfraction of these galaxies, for which we introduce the term ``dEdi,''reaches more than 50% at the bright end of the dE population anddecreases to less than 5% for magnitudes mB>16. Althoughpart of this observed decline might be due to the lower signal-to-noiseratio at fainter magnitudes, we show that it cannot be caused solely bythe limitations of our detection method. The luminosity function of ourfull dE sample can be explained by a superposition of dEdis and ordinarydEs, strongly suggesting that dEdis are a distinct type of galaxy. Thisis supported by the projected spatial distribution: dEdis show basicallyno clustering and roughly follow the spatial distribution of spirals andirregulars, whereas ordinary dEs are distributed similarly to thestrongly clustered E/S0 galaxies. While the flattening distribution ofordinary dEs is typical for spheroidal objects, the distribution ofdEdis is significantly different and agrees with their being flat oblateobjects. We therefore conclude that the dEdis are not spheroidalgalaxies that just have an embedded disk component but are instead apopulation of genuine disk galaxies. Several dEdis display well-definedspiral arms with grand-design features that clearly differ from theflocculent, open arms typical for late-type spirals that have frequentlybeen proposed as progenitors of dEs. This raises the question of whatprocess is able to create such spiral arms-with pitch angles like thoseof Sab/Sb galaxies-in bulgeless dwarf galaxies.

The ACS Virgo Cluster Survey. X. Half-Light Radii of Globular Clusters in Early-Type Galaxies: Environmental Dependencies and a Standard Ruler for Distance EstimationWe have measured half-light radii, rh, for thousands ofglobular clusters (GCs) belonging to the 100 early-type galaxiesobserved in the ACS Virgo Cluster Survey and the elliptical galaxy NGC4697. An analysis of the dependencies of the measured half-light radiion both the properties of the GCs themselves and their host galaxiesreveals that, in analogy with GCs in the Galaxy but in a milder fashion,the average half-light radius increases with increasing galactocentricdistance or, alternatively, with decreasing galaxy surface brightness.For the first time, we find that the average half-light radius decreaseswith the host galaxy color. We also show that there is no evidence for avariation of rh with the luminosity of the GCs. Finally, wefind in agreement with previous observations that the averagerh depends on the color of GCs, with red GCs being ~17%smaller than their blue counterparts. We show that this difference isprobably a consequence of an intrinsic mechanism, rather than projectioneffects, and that it is in good agreement with the mechanism proposed byJordán. We discuss these findings in light of two simple picturesfor the origin of the rh of GCs and show that both lead to abehavior in rough agreement with the observations. After accounting forthe dependencies on galaxy color, galactocentric radius, and underlyingsurface brightness, we show that the average GC half-light radii can be successfully used as a standard ruler fordistance estimation. We outline the methodology, provide a calibrationfor its use, and discuss the prospects for this distance estimator withfuture observing facilities. We find =2.7+/-0.35 pcfor GCs with (g-z)=1.2 mag in a galaxy with color(g-z)gal=1.5 mag and at an underlying surface z-bandbrightness of μz=21 mag arcsec-2. Using thistechnique, we place an upper limit of 3.4 Mpc on the 1 σline-of-sight depth of the Virgo Cluster. Finally, we examine the formof the rh distribution for our sample galaxies and provide ananalytic expression that successfully describes this distribution.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.

A Comparison of Surface Brightness Profiles for Ultracompact Dwarfs and Dwarf Elliptical Nuclei: Implications for the ``Threshing'' ScenarioUsing imaging from the Hubble Space Telescope, we derive surfacebrightness profiles for ultracompact dwarfs in the Fornax Cluster andfor the nuclei of dwarf elliptical galaxies in the Virgo Cluster.Ultracompact dwarfs are more extended and have higher surfacebrightnesses than typical dwarf nuclei, while the luminosities, colors,and sizes of the nuclei are closer to those of Galactic globularclusters. This calls into question the production of ultracompact dwarfsvia ``threshing,'' whereby the lower surface brightness envelope of adwarf elliptical galaxy is removed by tidal processes, leaving behind abare nucleus. Threshing may still be a viable model if the relativelybright Fornax ultracompact dwarfs considered here are descended fromdwarf elliptical galaxies whose nuclei are at the upper end of theirluminosity and size distributions.

Central Blue Clumps in Elliptical Galaxies of the Hubble Ultra Deep FieldElliptical galaxies larger than 10 pixels in the Hubble Ultra Deep Field(UDF) were surveyed for internal structure; 30 out of 100 in a sample of884 morphologically classified galaxies exhibit large blue clumps neartheir centers. Unsharp-masked images of the clearest cases arepresented. The distributions of the clumps on color-color andcolor-magnitude diagrams are about the same as the distributions ofisolated objects in the UDF with the same size, suggesting a possibleaccretion origin. In the few cases for which redshifts have beenpublished, the clump masses and star formation ages were determined fromstellar evolution models, as were the galaxy masses. The clump massscales with galaxy mass, probably because of selection effects, andranges from 106 to 108 Msolar forgalaxies with masses from 109 to 1011Msolar. The clump star formation age ranges between107 and 2×108 yr. With partial evaporationand core contraction in the intervening years, some of these clumpscould resemble globular clusters today. Stars that evaporate willcontribute to the field population in the elliptical galaxies.

The ACS Virgo Cluster Survey. II. Data Reduction ProceduresThe ACS Virgo Cluster Survey is a large program to carry out multicolorimaging of 100 early-type members of the Virgo Cluster using theAdvanced Camera for Surveys (ACS) on the Hubble Space Telescope. DeepF475W and F850LP images (~SDSS g and z) are being used to study thecentral regions of the program galaxies, their globular cluster systems,and the three-dimensional structure of Virgo itself. In this paper, wedescribe in detail the data reduction procedures used for the survey,including image registration, drizzling strategies, the computation ofweight images, object detection, the identification of globular clustercandidates, and the measurement of their photometric and structuralparameters.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.

The ACS Virgo Cluster Survey. I. Introduction to the SurveyThe Virgo Cluster is the dominant mass concentration in the LocalSupercluster and the largest collection of elliptical and lenticulargalaxies in the nearby universe. In this paper, we present anintroduction to the ACS Virgo Cluster Survey: a program to image, in theF475W and F850LP bandpasses (~Sloan g and z), 100 early-type galaxies inthe Virgo Cluster using the Advanced Camera for Surveys on the HubbleSpace Telescope. We describe the selection of the program galaxies andtheir ensemble properties, the choice of filters, the field placementand orientation, the limiting magnitudes of the survey, coordinatedparallel observations of 100 ``intergalactic'' fields with WFPC2, andsupporting ground-based spectroscopic observations of the programgalaxies. In terms of depth, spatial resolution, sample size, andhomogeneity, this represents the most comprehensive imaging survey todate of early-type galaxies in a cluster environment. We brieflydescribe the main scientific goals of the survey, which include themeasurement of luminosities, metallicities, ages, and structuralparameters for the many thousands of globular clusters associated withthese galaxies, a high-resolution isophotal analysis of galaxiesspanning a factor of ~450 in luminosity and sharing a commonenvironment, the measurement of accurate distances for the full sampleof galaxies using the method of surface brightness fluctuations, and adetermination of the three-dimensional structure of Virgo itself.ID="FN1"> 1Based on observations with the NASA/ESA Hubble SpaceTelescope obtained at the Space Telescope Science Institute, which isoperated by the association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.

Distances, Metallicities, and Ages of Dwarf Elliptical Galaxies in the Virgo Cluster from Surface Brightness FluctuationsWe have employed FORS1 and 2 at the Very Large Telescope at the EuropeanSouthern Observatory (ESO) to acquire deep B and R-band CCD images of 16dwarf elliptical (dE) galaxies in the direction of the Virgo cluster.For each dwarf, we measure the apparent R-band surface brightnessfluctuation (SBF) magnitude mR and the (B-R)0color in a number of fields at different galactocentric distances. Fromthe field-to-field variation of the two quantities, we determine the SBFdistance by means of the (B-R)0-MR relation. Thederived distances of the dwarfs range from 14.9 to 21.3 Mpc, with a mean1 σ uncertainty of 1.4 Mpc or 8% of the distance, which confirmsthat there is considerable depth in the distance distribution ofearly-type cluster members. For VCC 1104 (IC 3388), our SBF distancemodulus of (m-M)SBF=31.15+/-0.19 (17.0+/-1.5 Mpc) is in goodagreement with the Harris et al result of(m-M)TRGB=30.98+/-0.19 mag (15.7+/-1.5 Mpc) based on HubbleSpace Telescope (HST) observations and the tip magnitude of the redgiant branch. Combining our results with existing distances for giantVirgo ellipticals, we identify two major galaxy concentrations in thedistance distribution: a broad primary clump around (M-m)=31.0 mag (15.8Mpc) and a narrow secondary clump around 31.33 mag (18.5 Mpc). Anadaptive kernel analysis finds the two concentrations to be significantat the 99% (2.5 σ) and 89% (1.6 σ) levels, respectively.While the near-side clump of Virgo early-type galaxies can be associatedwith the subcluster centered on M87, the second clump is believed to bemainly due to the far side infalling group of galaxies around M86. Theages and metallicities of the dE stellar populations are estimated bycombining the observed (B-R)0 colors with Worthey's stellarpopulation synthesis models. It appears that the Virgo dE galaxies covera wider range in metallicity, from [Fe/H]~-1.4 (VCC 0815) to -0.5 (NGC4415), than Fornax cluster dEs. The derived metallicities place theVirgo dEs on the extension of the metallicity-luminosity relationdefined by the low-luminosity Local Group dEs. The data further suggestan age range from genuinely old (~17 Gyr) stellar systems such as IC3019 and IC 0783 to intermediate-age (8-12 Gyr) dwarfs such as NGC 4431and IC 3468.

Galaxy Populations and Evolution in Clusters. IV. Deep H I Observations of Dwarf Elliptical Galaxies in the Virgo ClusterIn this paper we present deep Arecibo H I and WIYN optical observationsof Virgo Cluster dwarf elliptical galaxies. Based on this data we arguethat a significant fraction of low-mass galaxies in the Virgo Clusterrecently underwent evolution. Our new observations consist of H I 21 cmline observations for 22 classified dE galaxies with optical radialvelocities consistent with membership in the Virgo Cluster. Clustermembers VCC 390 and VCC 1713 are detected with H I massesMHI=6×107 and 8×107Msolar, respectively, while MHI values in theremaining 20 dE galaxies have upper limits as low as~5×105 Msolar. We combine our results withthose for 26 other Virgo Cluster dE galaxies with H I observations inthe literature, seven of which have H I detection claims. New opticalimages from the WIYN telescope of five of these H I-detected dEgalaxies, along with archival data, suggest that seven of the claimeddetections are true H I detections, yielding a ~15% detection rate.These H I-detected, classified dE galaxies are preferentially locatednear the periphery of the Virgo Cluster. Three Virgo dE galaxies haveobserved H I velocity widths greater than 200 km s-1,possibly indicating the presence of a large dark matter content ortransient extended H I. We discuss the possible origins of these objectsand argue that they originate from field galaxies accreted onto highangular momentum orbits by Virgo in the last few Gyr. As a result ofthis, we argue, these galaxies are slowly transformed within the clusterby gradual gas-stripping processes, associated truncation of starformation, and passive fading of stellar populations. Low-mass,early-type cluster galaxies are therefore currently being produced asthe product of cluster environmental effects. We utilize our results ina simple model to estimate the recent (past 1-3 Gyr) average massaccretion rate into the Virgo Cluster, deriving a value of M~50Msolar yr-1.

Internal Dynamics, Structure, and Formation of Dwarf Elliptical Galaxies. II. Rotating versus Nonrotating DwarfsWe present spatially resolved internal kinematics and stellar chemicalabundances for a sample of dwarf elliptical (dE) galaxies in the VirgoCluster observed with the Keck telescope and Echelle Spectrograph andImager. In combination with previous measurements, we find that four outof 17 dE's have major-axis rotation velocities consistent withrotational flattening, while the remaining dE's have no detectablemajor-axis rotation. Despite this difference in internal kinematics,rotating and nonrotating dE's are remarkably similar in terms of theirposition in the fundamental plane, morphological details, stellarpopulations, and local environment. We present evidence for (or confirmthe presence of) faint underlying disks and/or weak substructure in afraction of both rotating and nonrotating dE's, but a comparable numberof counterexamples exist for both types that show no evidence of suchstructure. Absorption line strengths were determined based on theLick/IDS system (Hβ, Mg b, Fe5270, and Fe5335) for the centralregion of each galaxy. We find no difference in the line-strengthindices, and hence stellar populations, between rotating and nonrotatingdE galaxies. The best-fitting mean age and metallicity for our same of17 dE's are 5 Gyr and [Fe/H]=-0.3 dex, respectively, with rms spreads of3 Gyr and 0.1 dex. The majority of dE's are consistent with solar[α/Fe] abundance ratios. By contrast, the stellar populations ofclassical elliptical galaxies are, on average, older, more metal-rich,and α-enhanced relative to our dE sample. The line strengths ofour dE's are consistent with the extrapolation of the line strengthversus velocity dispersion trend seen in classical elliptical galaxies.Finally, the local environments of both rotating and nonrotating dE'sappear to be diverse in terms of their proximity to larger galaxies inreal or velocity space within the Virgo Cluster. Thus, rotating andnonrotating dE's are remarkably similar in terms of their structure,stellar content, and local environments, presenting a significantchallenge to theoretical models of their formation.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.

Hubble Space Telescope Detection of Spiral Structure in Two Coma Cluster Dwarf GalaxiesWe report the discovery of spiral-like structure in Hubble SpaceTelescope images of two dwarf galaxies (GMP 3292 and GMP 3629) belongingto the Coma Cluster. GMP 3629 is the faintest such galaxy detected in acluster environment, and it is the first such galaxy observed in thedense Coma Cluster. The large bulge and the faintness of the broadspiral-like pattern in GMP 3629 suggest that its disk may have beenlargely depleted. We may therefore have found an example of the``missing link'' in theories of galaxy evolution that have predictedthat dwarf spiral galaxies, particularly in clusters, evolve into dwarfelliptical galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555.

Star Formation Histories of Early-Type Galaxies. I. Higher Order Balmer Lines as Age IndicatorsWe have obtained blue integrated spectra of 175 nearby early-typegalaxies, covering a wide range in galaxy velocity dispersion andemphasizing those with σ<100 km s-1. Galaxies havebeen observed both in the Virgo Cluster and in lower densityenvironments. The main goals are the evaluation of higher order Balmerlines as age indicators and differences in stellar populations as afunction of mass, environment, and morphology. In this first paper, ouremphasis is on presenting the methods used to characterize the behaviorof the Balmer lines through evolutionary population synthesis models.Lower σ galaxies exhibit a substantially greater intrinsicscatter, in a variety of line-strength indicators, than do higherσ galaxies, with the large intrinsic scatter setting in below aσ of 100 km s-1. Moreover, a greater contrast inscatter is present in the Balmer lines than in the lines of metalfeatures. Evolutionary synthesis modeling of the observed spectralindexes indicates that the strong Balmer lines found primarily among thelow-σ galaxies are caused by young age, rather than by lowmetallicity. Thus we find a trend between the population age and thecentral velocity dispersion, such that low-σ galaxies have youngerluminosity-weighted mean ages. We have repeated this analysis usingseveral different Balmer lines and find consistent results from onespectral indicator to another.

VLT surface photometry and isophotal analysis of early-type dwarf galaxies in the Virgo clusterWe have carried out surface photometry and an isophotal analysis for asample of 25 early-type dwarf (dE and dS0) galaxies in the Virgo clusterbased on CCD images taken at the VLT with FORS1 and FORS2. For eachgalaxy we present B and R-band surface brightness profiles, as well asthe radial colour (B-R) profile. We give total apparent BR magnitudes,effective radii, effective surface brightnesses and total colourindices. The light profiles have been fitted with Sérsic modelsand the corresponding parameters are compared to the ones for otherclasses of objects. In general, dEs and dS0s bridge the gap in parameterspace between the giant ellipticals and the low-luminosity dwarfspheroidals in the Local Group, in accordance with previous findings.However, the observed profiles of the brightest cluster dwarfs showsignificant deviations from a simple Sérsic model, indicatingthat there is more inner structure than just a nucleus. This picture isreinforced by our isophotal analysis where complex radial dependenciesof ellipticity, position angle, and isophotal shape parameter a_4 areexhibited not only by objects like IC 3328, for which the presence of adisk component has been confirmed, but by many apparently normal dEs aswell. In addition, we find a relation between the effective surfacebrightness, at a given luminosity, and the strength of the offset of thegalaxy's nucleus with respect to the center of the isophotes. Dwarfswith large nuclear offsets also tend to have stronger isophotal twists.However, such twists are preferentially found in apparently round(epsilon < 0.3) galaxies and are always accompanied by significantradial changes of the ellipticity, which clearly points to a projectioneffect. In sum, our findings suggest the presence of substructure inmost, and preferentially in the less compact, bright early-type dwarfs.The physical (dynamical) meaning of this has yet to be explored.Based on observations collected at the European Southern Observatory(ESO 63.O-0055 and 65.N-0062).Figure \ref{fig1} is only available in electronic form athttp://www.edpsciences.org

Embedded disks in Fornax dwarf elliptical galaxiesWe present photometric and kinematic evidence for the presence ofstellar disks, seen practically edge-on, in two Fornax dwarf galaxies,FCC 204 (dS0(6)) and FCC 288 (dS0(7)). This is the first time suchstructures have been identified in Fornax dwarfs. FCC 288 has only asmall bulge and a bright flaring and slightly warped disk that can betraced out to +/- 23 arcsec from the center (2.05 kpc for H_0=75 kms-1 Mpc-1). FCC 204's disk can be traced out to+/-20 arcsec (1.78 kpc). This galaxy possesses a large bulge. Theseresults can be compared to the findings of Jerjen et al. (\cite{jer00})and Barazza et al. (\cite{bar02}) who discovered nucleated dEs withspiral and bar features in the Virgo Cluster.Based on observations collected at the European Southern Observatory,Chile (ESO Large Programme Nr. 165.N-0115).

The UZC-SSRS2 Group CatalogWe apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.

More evidence for hidden spiral and bar features in bright early-type dwarf galaxiesFollowing the discovery of spiral structure in IC 3328 (Jerjen et al.\cite{Jerjen2000}), we present further evidence that a sizable fractionof bright early-type dwarfs in the Virgo cluster are genuine diskgalaxies, or are hosting a disk component. Among a sample of 23nucleated dwarf ellipticals and dS0s observed with the Very LargeTelescope in B and R, we found another four systems exhibitingnon-axisymmetric structures, such as a bar and/or spiral arms,indicative of a disk (IC 0783, IC 3349, NGC 4431, IC 3468). Particularlyremarkable are the two-armed spiral pattern in IC 0783 and the bar andtrailing arms in NGC 4431. For both galaxies the disk nature hasrecently been confirmed by a rotation velocity measurement (Simien &Prugniel \cite{Simien2002}). Our photometric search is based on aFourier decomposition method and a specific version of unsharp masking.Some ``early-type'' dwarfs in the Virgo cluster seem to be formerlate-type galaxies which were transformed to early-type morphology, e.g.by ``harassment'', during their infall to the cluster, while maintainingpart of their disk structure. Based on observations collected at theEuropean Southern Observatory, Chile.

Kinematical data on early-type galaxies. VI.We present the result of spectroscopic observations of a sample of 73galaxies, completing the database published in this series of articles.The sample contains mostly low-luminosity early-type objects, includingfour dwarfs of the Local Group (in particular, deep spectra of NGC 205),15 dEs or dS0s in the Virgo cluster, and UGC 05442, a spheroidal dwarfof the M 81 group. We have measured the central velocity dispersion forall but one object, and determined the major-axis rotation andvelocity-dispersion profiles for 59 objects. For the current sample ofdiffuse (or dwarf) elliptical galaxies, we have compared stellarrotation to velocity dispersion; the analysis suggests that theseobjects may be nearly rotationally flattened, and therefore thatanisotropy may be less important than previously thought. Based onobservations collected at the Observatoire de Haute-Provence. Table 1 isalso, and Tables 2 and 4 only, available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/384/371

Star Formation Histories of Nearby Dwarf GalaxiesProperties of nearby dwarf galaxies are briefly discussed. Dwarfgalaxies vary widely in their star formation histories, the ages oftheir subpopulations, and in their enrichment history. Furthermore, manydwarf galaxies show evidence for spatial variations in their starformation history; often in the form of very extended old populationsand radial gradients in age and metallicity. Determining factors indwarf galaxy evolution appear to be both galaxy mass and environment. Wemay be observing continuous evolution from low-mass dwarf irregulars viatransition types to dwarf spheroidals, whereas other evolutionarytransitions seem less likely.

Galaxy Populations and Evolution in Clusters. I. Dynamics and the Origin of Low-Mass Galaxies in the Virgo ClusterEarly-type dwarfs are the most common galaxy in the local universe, yettheir origin and evolution remain a mystery. Various cosmologicalscenarios predict that dwarf-like galaxies in dense areas are the firstto form and hence should be the oldest stellar systems in clusters. Byusing radial velocities of early-type dwarfs in the Virgo cluster wedemonstrate that these galaxies are not an old cluster population buthave signatures of production from the infall of field galaxies.Evidence of this includes the combined large dispersions andsubstructure in spatial and kinematic distributions for Virgo early-typedwarfs and a velocity dispersion ratio with giant ellipticals expectedfor virialized and accreted populations. We also argue that thesegalaxies cannot originate from accreted field dwarfs, but must havephysically evolved from a precursor population, of different morphology,that fell into Virgo some time in the past.

The Frequency of Active and Quiescent Galaxies with Companions: Implications for the Feeding of the NucleusWe analyze the idea that nuclear activity, either active galactic nuclei(AGNs) or star formation, can be triggered by interactions by studyingthe percentage of active, H II, and quiescent galaxies with companions.Our sample was selected from the Palomar survey and avoids selectionbiases faced by previous studies. This sample was split into fivedifferent groups, Seyfert galaxies, LINERs, transition galaxies, H IIgalaxies, and absorption-line galaxies. The comparison between the localgalaxy density distributions of the different groups showed that in mostcases there is no statistically significant difference among galaxies ofdifferent activity types, with the exception that absorption-linegalaxies are seen in higher density environments, since most of them arein the Virgo Cluster. The comparison of the percentage of galaxies withnearby companions showed that there is a higher percentage of LINERs,transition galaxies, and absorption-line galaxies with companions thanSeyfert and H II galaxies. However, we find that when we consider onlygalaxies of similar morphological types (elliptical or spiral), there isno difference in the percentage of galaxies with companions amongdifferent activity types, indicating that the former result was due tothe morphology-density effect. In addition, only small differences arefound when we consider galaxies with similar Hα luminosities. Thecomparison between H II galaxies of different Hα luminositiesshows that there is a significantly higher percentage of galaxies withcompanions among H II galaxies with L(Hα)>1039 ergss-1 than among those with L(Hα)<=1039ergs s-1, indicating that interactions increase the amount ofcircumnuclear star formation, in agreement with previous results. Thefact that we find that galaxies of different activity types have thesame percentage of companions suggests that interactions betweengalaxies is not a necessary condition to trigger the nuclear activity inAGNs. We compare our results with previous ones and discuss theirimplications.

1.65 μm (H-band) surface photometry of galaxies. VII. Dwarf galaxies in the Virgo ClusterWe present near-infrared H-band (1.65 μm) observations and surfacebrightness profile decompositions for 75 faint (13.5 5)is a strong, non-linear function of the total luminosity. We also findthat: i) among dE and dS0 galaxies D profiles are absent; 50% of thedecompositions are of type M, the remaining being of type E or T. ii)Less than 50% of the giant elliptical galaxies have pure D profiles, themajority being represented by M profiles. iii) Most giant galaxies (fromelliptical to Sb) have M profiles. iv) Most of late type spirals (Scd toBCD) have either E or T profiles. v) The type of decomposition is astrong function of the total H band luminosity, independent of theHubble classification: the fraction of type E decompositions decreaseswith increasing luminosity, while those of type M increase withluminosity. Pure D profiles are absent in the low luminosity rangeLH<1010 Lsolar and become dominantabove 1011 Lsolar, while T profiles are presentonly among low luminosity galaxies. vi) We find that dE-peculiargalaxies have structural parameters indistinguishable from those oflate-type dwarfs, thus they might represent the missing link between dEsand dIs. Based on observations taken with the ESO/NTT (ESO program64.N-0288), with the Telescopio Nazionale Galileo (TNG) operated on theisland of La Palma by the Centro Galileo Galilei of the CNAA at theSpanish Observatorio del Roque de los Muchachos of the IAC, with the SanPedro Martir 2.1~m telescope of the Observatorio Astronomico Nacional(OAN, Mexico), and with the OHP 1.2~m telescope, operated by the FrenchCNRS.

Off-center nuclei in dwarf elliptical galaxiesWe have searched for off-center nuclei in 78 ``nucleated'' dwarfelliptical (dE,N) galaxies, drawing on digitized photographic imagesfrom a previous study of Virgo cluster dwarfs. The search is based on asimple algorithm which compares the center coordinates of a series ofouter elliptical isophotes with the position of the galaxy's nucleus.Monte Carlo simulations of the measuring procedure are used to assessrandom and systematic errors. Roughly 20% of all dwarf nuclei in thesample (neglecting uncertain cases) are found to be significantlyoff-centered. The typical displacement is 1arcsec , or 100 pc (assuminga Virgo cluster distance of 20 Mpc), corresponding to 0.5 to 1 effectiveradii of the dwarf galaxy. There is a tendency of the nuclear off-centerdisplacement to increase with decreasing surface brightness of theunderlying galaxy. A similar trend was found with normal ellipticalgalaxies before. If real, the effect could mean that a nucleus canoscillate about the galaxy center with larger amplitude in a shallower(less cuspy) gravitational potential. In an appendix we present evidencefor the existence of a strong, unambiguous relation between the nuclearmagnitude and the ellipticity of dE,N galaxies. If a nucleus iscomprising 4% or more of the total light of the underlying galaxy, thatgalaxy is nearly always round, i.e. ellipticity less than 0.15 (dE0,dE1). This effect was predicted qualitatively long ago as the result ofbox orbit disruption caused by a central massive compact object (blackhole).

IC3328: A ``dwarf elliptical galaxy'' with spiral structureWe present the 2-D photometric decomposition of the Virgo galaxyIC3328. The analysis of the global light distributionof this morphologically classified nucleated dwarf elliptical galaxy(dE1,N) reveals a tightly wound, bi-symmetric spiral structure with adiameter of 4.5 kpc, precisely centered on the nucleus of the dwarf. Theamplitude of the spiral is only three percent of the dwarf's surfacebrightness making it the faintest and smallest spiral ever found in agalaxy. In terms of pitch angle and arm winding the spiral is similar tothe intermediate-type galaxy M51, but it lacks the dust and prominent HIi regions which signal the presence of gas. The visual evidence of aspiral pattern in an early-type dwarf galaxy reopens the question onwhether these dwarfs are genuine rotationally supported or anisotropicstellar systems. In the case of IC3328, we argue for a nearly face-ondisk (dS0) galaxy with an estimated maximum rotation velocity ofvc,max~ 55 km s-1 . The faintness of the spiraland the small motions within it, suggests that we could be seeingswing-amplified noise. The other possibility is a tidal origin, causedby the near passage of a small companion. Based on observationscollected at the European Southern Observatory (ESO 63.O-0055)

Total magnitude, radius, colour indices, colour gradients and photometric type of galaxiesWe present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.

Is the shape of the luminosity profile of dwarf elliptical galaxies an useful distance indicator?The shape of the surface brightness profile of dE galaxies, quantifiedby parameter n of Sersic's generalized profile law, has recently beenput forward as new extragalactic distance indicator (Young & Currie1994). Its application to the Virgo cluster has subsequently led to theclaim that the Virgo dEs are not lying in the cluster core but aredistributed in a prolate structure stretching from 8 to 20 Mpc distance(Young & Currie 1995). This claim is refuted here. We have fitted aSersic law to the surface brightness profiles of 128 Virgo cluster dEsand dS0s from the photometry of Binggeli & Cameron (1991). Thedispersion of the n - M relation is indeed large (sigma_rms ~ 0.9 mag).However, we argue that this scatter is not due to the depth of the Virgocluster, but is essentially intrinsic. Contrary to what one would expectfrom the cluster depth hypothesis, there is no clearvelocity-``distance'' relation for a sample of 43 Virgo dEs and dS0swith known redshifts. The analysis of Young & Currie (1995) ishampered by the use of low-resolution photometry and flawed by theassumption that the n - M and n - R relations can be used independently.By combining different Sersic law parameters, the scatter of the scalingrelations can be reduced somewhat, but never below sigma_rms ~ 0.7 mag,at least for the Virgo cluster. For the purpose of distancemeasurements, this falls short of the well-established Tully-Fisher andD_n - sigma methods, and it is comparable to what one can get alreadyfrom the < mu >_eff - M relation for dEs, which does not requireany profile modelling.

An image database. II. Catalogue between δ=-30deg and δ=70deg.A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.

The fundamental plane of early-type galaxies: stellar populations and mass-to-light ratio.We analyse the residuals to the fundamental plane (FP) of ellipticalgalaxies as a function of stellar-population indicators; these are basedon the line-strength parameter Mg_2_ and on UBVRI broad-band colors, andare partly derived from new observations. The effect of the stellarpopulations accounts for approximately half the observed variation ofthe mass-to-light ratio responsible for the FP tilt. The residual tiltcan be explained by the contribution of two additional effects: thedependence of the rotational support, and possibly that of the spatialstructure, on the luminosity. We conclude to a constancy of thedynamical-to-stellar mass ratio. This probably extends to globularclusters as well, but the dominant factor would be here the luminositydependence of the structure rather than that of the stellar population.This result also implies a constancy of the fraction of dark matter overall the scalelength covered by stellar systems. Our compilation ofinternal stellar kinematics of galaxies is appended.

A Catalog of Stellar Velocity Dispersions. II. 1994 UpdateA catalog of central velocity dispersion measurements is presented,current through 1993 September. The catalog includes 2474 measurementsof 1563 galaxies. A standard set of 86 galaxies is defined, consistingof galaxies with at least three reliable, concordant measurements. It issuggested that future studies observe some of these standard galaxies sothat different studies can be normalized to a consistent system. Allmeasurements are reduced to a normalized system using these standards.

Near Infrared Imaging of Dwarf Ellipticals Irregulars and Blue Compact Galaxies in the Virgo ClusterNew near-IR images are presented for 13 dwarf galaxies in the Virgocluster. Together with previous data these provide a data base of JHKimaging for 26 dwarf ellipticals (dEs), dwarf irregulars (dIs) and bluecompact dwarfs (BCDs). These images show the dIs to be highly asymmetricand unrelaxed, implying that they are dynamically young and unevolved.This is consistent with their blue near-IR and optical-IR colours whichare most easily explained by young stellar populations. The dEs aresymmetrical and apparently relaxed, with very uniform colours indicatingthat they are dominated by old stars. They generally have exponentiallight profiles, but the brighter galaxies tend to exhibit more cuspedlight distributions, similar to the de Vaucouleurs profiles of brightellipticals. The BCDs have moderately asymmetric light profiles, andparadoxically red colours, possibly indicating an intermediate-agestellar population. They are probably dEs which have undergone bursts ofstar formation in the last few X 10^9^ years, whilst the dIs are afundamentally distinct population. Colour gradients are present in manyof the galaxies, invariably in the sense that the nuclei are redder thanthe surrounding galaxy light.